[lug] Over my Head in a Puddle

[Bill Thoen]

Hi folks,

I have a few Centos 5.5 machines that used to talk to each other, but I've inherited this system from a real sysadmin, and he set up plenty of things I just don't know anything about. When they worked it was great, but now after moving the whole system to a small service room and enduring a month of cement dust in the air, then putting it all back together... Well, some stuff doesn't work.

What I would like to know is how do go from "network is unreachable" to a working system? There's so much in place I dont know where to start or what to tweak, and my friend who built it has moved to Texas and I'm trying to learn how he did it. I'm hoping someone here can give me some advice on how to sort out issues between switches and cabling, firewalls in hardware, how to discover where the network problems are., etc.
So... Heres the situation. what's been subtracted is a very nice T1, replaced by a wireless modem with built in switch, router, firewall, DSL, gateway, etc. From Century Link. 

My wireless laptop with Windows can talk to it no problem. But it looks to me like a few quick changes here and I will have hosed the system totally, so I fear it.

Next I have my main server, a Centos 5.5 Linux box with dovecot, postfix, amavis, spam assassin,  and I don't know what else without looking. It is managing mail just fine, but when I try to get in via ssh on my laptop, I get a network timeout. Even though my laptop is wireless, it depends on the modem for a dhcp address, and so I suppose I use the 192.168.0.** address instead of the 67.42.196.***. but doesnt matter. Neither will let me in. But when I'm at home I can ssh to this machine any time I want.

If you start with the server, what's the first things you check when you get a "network is unreachable" error?  I try ifconfig and get what looks like a good result. Eth0 is showing that it's attached to the LAN address, eth1 has the right Internet IP,  lo is happy with 127.0.0.1. So no real information there. But I do have another machine with 1 nic that's not showing an ip, so I have one problem there.

I also know a little about ping basics. From the server Ping 127.0.0.1 works. Ping the server on the same machine works too. Ping to a working machine in the LAN works, and ping to an outside computer works. From a different machine in the LAN, pinging the server results in a mixed bag. The first response say it's redirecting to the ip I gave it. It looks like it went out to the DNS servers snd came back with yhe same address. Then the next 3 response are complete failures, returning errors of  "destination host unreachable" on the 5th, 6th and beyond it works fine.

So I've got problems with problems.  But the big problem is that some of these seem to be related, some are independent, and I'm sure but somewhere in the network there are emergent problems. If someone could tell mer what software to use for basic network triage, I can RTFM,  and then ask more directed questions, but I don't even know what I'm looking for at this point. What I would like first is to get my server straightened out.  

Regards,

Bill Thoen
GISnet
http://gisnet.com
303-786-9961